The Origins of Genome Complexity
Monday, October 20, 2003 - 9:30am - 10:20am
Michael Lynch (Indiana University)
Complete genomic sequences from diverse phylogenetic lineages reveal striking increases in genome complexity across the prokaryote to unicellular eukaryote to multicellular eukaryote boundaries. The changes include gradual growth in gene number resulting from the retention of duplicate genes, more abrupt increases in the abundance of spliceosomal introns and mobile genetic elements, and enhanced modularity of gene regulation. A case can be made that many of these changes emerged passively in response to substantial long-term population-size reductions that accompanied increases in organism size and magnified the power of random genetic drift. Under this model, much of the restructuring of eukaryotic genome organization and the roots of many aspects of organismal complexity were initiated by nonadaptive processes. Although the mutational changes necessary for genomic modification are initiated by molecular processes, the population-genetic environment ultimately defines the permissible paths of evolution. The simple genomes of most microbial species can be understood in this context, without invoking direct selection for streamlined genomes, and direct selection for complexity need not be invoked to explain genomic expansion in multicellular species.